The first turbocharger was proposed by Alfred Buchi in 1905. The turbocharger began as a device to capture energy that was wasted in the exhaust system of a naturally aspirated engine by passing exhaust gases over a series of blades on a turbine wheel to produce a rotary power output. The turbine was connected to and drove a compressor to increase the charge density of an engine and therefore the power output. Since 1905, immense strides have been made in the turbo machinery field to improve the efficiency, flexibility and durability of individual components. A key element in the quest for efficiency is the turbine. In the most common commercial form, the turbine consists of a centripetal turbine wheel having a radial entry receiving heated gases from a turbine nozzle and accelerating them and extracting energy to drive either a compressor or other power load device. Strides have been made in metallurgy to enable higher operating temperatures and higher rpm's. Concurrent with these advances was the attempt to operate the turbine at maximum efficiency over a range of operating conditions.
A major impediment to maintaining efficiency over a broader range of operating conditions is a result of the incident angle with which the exhaust or heated gas flow is introduced to the periphery of the turbine wheel. The turbine is typically designed for maximum efficiency at full load conditions at rated speed. However, when the load drops off to for example 25%, a turbine with a rated efficiency of 80% can easily drop to an efficiency of 25%. This is primarily due to an incident angle that is not optimum for the part throttle conditions. The drop in efficiency significantly impacts part load performance characteristics of an engine system with which it is used and may ultimately influence the ability to flow air required for clean combustion characteristics.
Attempts have been made to accommodate this condition by incorporating a ring of variable angle vanes around the periphery of the turbine wheel. The vanes are pivoted to change their angle with respect to the wheel and to vary the throat area for gases entering the wheel to increase gas velocity during part load engine conditions. However, such systems have been proven to questionable reliability, given the carbon laden nature of the gases passing through it and the requirement for a large plurality of vanes to be pivoted in precise unison.
Therefore, what is needed in the art is a simple and effective power turbine having a broad range of efficiency characteristics.